Patterns of Mos expression and MPF activity during oocyte meiotic maturation and at fertilization in Xenopus, mouse, and starfish. Prophase I-arrested oocytes require a physiological stimulus to undergo meiotic maturation: progesterone in frogs, release from the follicle in mammals, and 1-methyl-adenine in starfish. Once activated, MPF promotes entry into the first meiotic division: breakdown of the nuclear envelope (GVBD for germinal vesicle breakdown) and formation of the metaphase I spindle (MI). MPF activity falls due to partial cyclin degradation at meiosis I/meiosis II transition or interkinesis (IK), during which chromosomes remain condensed without nuclear membranes and in the absence of DNA replication. MPF rises again leading to entry into meiosis II. In vertebrates, oocytes arrest at metaphase II (MII), while in echinoderms, oocytes complete the second meiotic division and arrest at the G1 phase. Mos translational timing is different among species, occurring before MPF activation in Xenopus (however, Mos protein is unstable until GVBD and MAPK activity is detected only at time of MPF activation, not illustrated) and during metaphase I in other species.